Instead of creating a pipeline GObject, just ask for the VkPipeline.
And instead of having the Op handle it, just let the renderpass look
up/create the relevant pipeline while creating commands so that it can
insert vkCmdBindPipeline calls as-needed.
This reverts most of commit f420c143e0
again because it turns out GPUs like combined images and samplers.
But: The one thing we don't revert is allowing the C code to select any
combination of sampler and image:
gsk_vulkan_render_get_image_descriptor() now takes a 2nd argument
specifying the sampler.
This allows the same flexibility as before, we just combine things
early.
This change was inspired by
https://developer.nvidia.com/blog/vulkan-dos-donts/
Instead of creating the op manually, just pass in the renderpass and
have the op created from there.
This way ops aren't really initialized anymore, they are more appended
to the queue, so instead of foo_op_init() we can just call the function
foo_op().
